This invention relates to electrostatic charging of flash spun polymer plexifilamentary film-fibril strands which have been spread apart to separate the fibrils, and wherein the electrostatic charging further separates the fibrils and helps to pin the strands down to a moving conveyor.
The process of forming plexifilamentary film-fibril strands and forming the same into non-woven sheet material has been disclosed and extensively discussed in U.S. Pat. No. 3,081,519 to Blades et al., U.S. Pat. No. 3,227,794 to R. D. Anderson et al., U.S. Pat. No. 3,169,899 to Steuber, U.S. Pat. No. 3,851,023 to Brethauer et al. and U.S. Pat. No. 3,387,326 to Hollberg et al. This process and various improvements thereof have been practiced for a number of years by E. I. du Pont de Nemours and Company (DuPont) in the manufacture of Tyvek(copyright) spunbonded olefin.
Part of the foregoing manufacturing process includes a step of applying an electrostatic charge to a flattened and partially spread open plexifilamentary film-fibril strand after it is spun at a spin pack and before it is laid down on a conveyor belt. The electrostatic charges thereby applied to the individual fibrils cause the fibrils to repel one another, thus separating themselves and further xe2x80x9copening-upxe2x80x9d the flattened strand (or probably more accurately described as a plexifilamentary film-fibril web once the strand has been flattened). The xe2x80x9copened-upxe2x80x9d web is then suited to being laid down, along with other webs from adjacent spin packs onto a conveyor to form a sheet. The conveyor may also be provided with an electrostatic charge opposite to the charge on the strand thereby causing the webs to be attracted to the conveyor and remain pinned down to the conveyor. The process of applying a charge to the webs has worked quite satisfactorily in the current arrangements, although the equipment for applying the charges continue to require improvements in a number of areas.
In spite of the success and satisfaction with the overall flash spinning process and system, the process includes the use of perchlorofluorocarbon (CFC) solvents which are currently believed to cause ozone depletion and the use of which will soon be legislatively foreclosed. Accordingly, alternative solvents having suitable performance characteristics in the flash-spinning process are being aggressively sought. DuPont has expended considerable resources developing alternative solvents and has focused on several that may eventually be used commercially. As might be expected, the different solvents require some modifications in the manufacturing process or present problems that did not exist using the CFC solvents.
Hydrocarbon solvents are currently considered the most attractive alternatives to the potentially ozone depleting solvents presently in use. However, the resulting hydrocarbon atmosphere, into which the strands are spun, causes a lower charge current efficiency for the electrostatic charge applying equipment. In other words, in the process of manufacturing flash spun polyolefins, the use of promising hydrocarbon solvents reduces the effective electrostatic charge applied to the web passing through the electrostatic field for a given current as compared to the same process using a conventional CFC solvent. As a result, the webs are not as fully opened up and the resulting non-woven sheet is less uniform than a sheet formed of more fully charged webs. Sheet uniformity is an important issue for product quality and has a substantial effect on the value of the product.
Although it would be logical to increase the current to the electrostatic charge applying equipment to thereby increase the charge applied to the web, increasing the amperage to the electrostatic charging system causes excessive deterioration of the current elastomeric target plates in the spin pack. Target plate deterioration would substantially reduce the duration for which the spin pack may be operational in a spin cell. Moreover, even if the deterioration of the target plates may be resolved (such as using a metal target plate), target plates do become fouled with polymer residue during the flash spinning process. The polymer coating reduces the charging efficiency and the electrostatic charging system responds by increasing the charging current to the ion gun to maintain the desired charge on the web. When the target plate is sufficiently fouled so as to require greater charging current than the system is designed to provide, the spin pack must be shut down to be replaced. Replacement of spin packs may require a production shutdown, so average operational life span of a spin pack may seriously effect the economics of production.
Accordingly, it is a primary object of the present invention to provide satisfactory electrostatic web charging performance of a flash spun plexifilamentary film-fibril web at acceptable charge currents and using an environmentally suitable solvent.
More specifically, it is a further object of the present invention to provide an environmentally acceptable solvent that is suited to high pressure dissolving of polyolefins and flash spinning the same wherein an adequate charge can be applied to the produced plexifilamentary film fibril strands so as to be suitable for laying down the webs and forming satisfactory non-woven sheet material on a moving conveyor.
It is a more particular object of the present invention to develop suitable additives for known potential solvents to provide environmentally acceptable solvent/spin liquids for making flash spun plexifilamentary film-fibril webs wherein the vapor from the spin liquid is electrically suitable for adequate charging of the webs so as to be suitable for laying down the webs and forming satisfactory non-woven sheet material on a moving conveyor.
The above and other objects have been attained by the present invention which is embodied by an improved process for flash-spinning plexifilamentary film-fibril strands of a fiber-forming crystalline polyolefin. The process comprises flash-spinning the plexifilamentary film-fibril strands at a temperature of 130xc2x0 C. to 300xc2x0 C. and at a mixing pressure greater than autogenous pressure from a solution consisting essentially of 8 to 35 weight percent of the polyolefin and 92 to 65 weight percent of a spin liquid. The spin liquid may be a saturated C4-C7 hydrocarbon or a mixture of a saturated C4-C7 hydrocarbon mixed with at least one cosolvent. After the strands are spun, the process includes electrostatically charging the strands and laying the strands as a sheet on a continuously moving surface. The improvement in the process comprises conducting the electrostatic charging step in an atmosphere comprising at least one charge-improving compound, predominantly in gas or vapor form, wherein the charge-improving compound belongs to one of two groups. The first group comprises compounds that have an atmospheric boiling temperature of less than 100xc2x0 C. and consists of one of carbon dioxide, hydrofluorocarbons, hydrochlorofluorocarbons, perfluorocarbons, C1-C4 alcohols, aliphatic ketones, and polar solvents. The second group consists of compounds not listed in the first group that are within the following categories of compounds: compounds of the types listed in the first group except having atmospheric boiling temperatures of at least 100xc2x0 C.; halogen gases; acid halides; halocarbons that are not listed in group A; hydroxylic compounds, ethers, carboxylic acids; esters; sulfur compounds; non-aliphatic ketones; aldehydes; nitro compounds; nitrogen oxides; nitrites; ammonia; amines; amides; and any halogenated derivatives of the above compounds which do not already contain a halogen atom. The atmosphere should have least 0.1 ppm of charge-improving compound, up to ten weight percent of the first group of charge-improving compounds, and less than seventy-five weight percent of the second group of charge-improving compounds.